A beam guiding apparatus for an extreme ultraviolet radiation (EUV) generating device has been disclosed in, for example, US 2011/0140008 A1. The beam guiding apparatus described therein serves for guiding laser radiation which was generated and amplified in a beam source, for example, a driver laser system. The beam guiding apparatus guides the laser beam from the driver laser system to a focusing apparatus in order to focus the laser beam in a target region. A target material which is converted into a plasma state when irradiated by the laser beam and which emits EUV radiation in the process is provided in the target region. The target material is typically provided in a vacuum chamber, into which the laser beam enters by way of an opening.
In general, some of the target materials (e.g. tin) evaporate when irradiated by the laser beam and deposit on the optical surfaces of optical elements arranged in the vicinity of the target region. In order to counteract this problem, a CO2 laser is generally used as a laser source or as a driver laser. As a result of the long wavelength (for example, approximately 10.6 μm), CO2 laser radiation is also reflected by optical elements that have a comparatively rough optical surface caused by, e.g., tin deposits. The use of a laser source or a driver laser in the form of a CO2 laser can lead to high conversion efficiency between the input power of the driver laser and the output power of the generated EUV radiation in the case of specific target materials such as, e.g., tin.
WO 2011/162903 discloses a driver laser arrangement that has a first seed laser for generating a pre-pulse and a second seed laser for generating a main pulse. The pre-pulse and the main pulse have different wavelengths and they are combined by a beam combiner in order to pass along a common beam path through an amplifier and the beam guiding apparatus following the driver laser arrangement. The pre-pulse can, for example, influence the target material. For example, the pre-pulse can heat up, expand, vaporize, and/or ionize the target material to generate a weak, or possibly a strong, plasma. The main pulse can, for example, convert the majority of the material influenced by the pre-pulse into the plasma state and generate EUV radiation in the process.
Both the pre-pulse and the main pulse are guided into the vacuum chamber through the same opening. Since the pressure inside the vacuum chamber is typically lower than the pressure in the beam guidance outside of the vacuum chamber, it is generally required to seal the opening in the vacuum chamber in a gas-tight manner from the surroundings by a transmissive optical element which, for example, may form a window, e.g., a plane plate. The transmissive optical element should enable a passage for both laser beams (pre-pulse and main pulse) with small losses and/or aberrations. However, if the wavelength of the pre-pulse differs significantly from the wavelength of the main pulse, the transmissive optical element can lead to losses and aberration in pre-pulse or main pulse or both.